Give tanks a 40 yard AOE LOS status that everyone who is friendly takes x% less damage but the tank takes x% more damage the more tanks would increase this status meaning there needs to be a balance of how many tanks too many and the tanks would die much quicker too little and the ones they want to protect risk dying quicker.
This would encourage people to stay near the tank and healers to heal the tank while disincentivising DPS to attack anyone other then the tank.